Due to the increasing concerns of environmental awareness, gradually single-use batteries (primary batteries) have been replaced by the high performance secondary batteries which are widely used in consumer electronic products, energy storage systems, and other industries.
Following the development of auto industry, the demands for lithium ion secondary batteries have increased. Besides the demands for favorable charging-discharging performances, the safety and battery life of lithium ion secondary batteries should also be taken into consideration. The development trend of lithium ion secondary batteries is moving toward developing power storage batteries for energy storage systems. In order to develop lithium ion secondary batteries to fulfill the system-scale requirements and satisfy the development trend of energy storage technologies, the battery capacitance of the lithium ion secondary battery has to reach the scale of MW/MWh, the cycle life of the lithium ion secondary battery used in a mobile phone should be over 2000 times and the cycle life of the lithium ion secondary battery used in an energy storage system should be over 6000 times.
A lithium ion secondary battery is produced by coiling an anode plate, a separation membrane, and a cathode plate, placing the anode plate, separation membrane and cathode plate into a container, adding an electrolyte and sealing the container, wherein the anode plate is consisting of a anode current collector constructed by a copper foil and an anode active material (such as a carbon-based material) which is coated on the surface of the anode current collector. The copper foil can be a rolled copper foil or an electrolytic copper foil. In addition, the electrolytic copper foil is manufactured by using the aqueous solution consisting of sulfuric acid and copper sulfate as an electrolyte, using a titanium plate coated by iridium element or iridium oxide as a dimensionally stable anode (DSA), using a titanium drum as a cathode, applying direct current between two electrodes to electrodeposit copper ions from the electrolyte on the titanium drum, peeling off the electrolytic copper foil from the surface of the titanium drum and continuously winding. The surface of the electrolytic copper foil that is contacting the surface of the titanium drum is called “shiny side (S side),” and the other side is called “matte side (M side).” In general, the roughness of the S side of the electrolytic copper foil is determined by the roughness of the surface of the titanium drum. Therefore, the roughness of the S side is more consistent, and the roughness of the M side is controlled by adjusting the condition of the copper sulfate electrolyte.
Based on the above, in order to reduce the roughness of the M side of the electrolytic copper foil, it is known that the crystalline phase of the electrolytic copper foil is changed by adding an organic additive, such as glue with a small molecular weight (e.g. gelatin), hydroxyethyl cellulose (HEC), or polyethylene glycol (PEG), and adding a sulfur-containing compound such as sodium 3-mercapto-1-propanesulfonate (MPS) or bis-(3-sodiumsulfopropyl disulfide (SPS) with the effect of reducing (refining) the size of the crystals to the copper sulfate electrolyte.
With respect to the crystalline phase of the electrolytic copper foil obtained by a conventional method disclosed in KR 10-1117370, the sum of the texture coefficients of a (111) surface and a (200) surface of the electrolytic copper foil is between 60-85% based on the total sum of the texture coefficients (TC) of the (111) surface, the (200) surface and a (220) surface, wherein the texture coefficient ranges of the (111) surface and the (200) surface of the electrolytic copper foil are between 18-38% and 15-40%, respectively, so as to obtain an electrolytic copper foil with fine crystalline grain structure, which a roughness of the M side is lower and suitable for use in a lithium ion secondary battery.
Therefore, traditionally, when the texture coefficients of a (111) surface and a (200) surface of an electrolytic copper foil are higher, the crystalline grains of the electrolytic copper foil become smaller, the roughness of the M side become lower, the tensile strength of the electrolytic copper foil become higher, and the elongation become lower. On the contrary, when the texture coefficients of a (111) surface and a (200) surface of an electrolytic copper foil are lower and/or the texture coefficients of a (220) surface and a (311) surface of an electrolytic copper foil are higher, the crystalline grains of the electrolytic copper foil become larger and the roughness of the M side become higher.
Based on the above, it is known that for the electrolytic copper foil prepared by a conventional method, it is predictable that the texture coefficients of the (111) surface and the (200) surface are higher, such that the roughness of the M side is lower and the electrical capacity is increased. However, the electrolytic copper foil will be easily fractured due to expansion and shrinkage and leads to the reduction of a battery life.
Therefore, the urgent issue is to develop an electrolytic copper foil applied in a lithium ion secondary battery that has an M side with a lower roughness and increases the cycle life of the battery.